I'm one of the users who has one Apple ID that I use for e-mail, calendar, etc., and a different ID that I use for iTunes and the App Store. Is one of these preferred (or required) for use as the Apple ID to use with Apple Card? Are there differences in the features available or user experience of choosing one over the other?

Which one of the two you use to sign into iCloud? The support document, How to apply for Apple Card states you'll need to be signed into iCloud with your Apple ID. This can be inferred to mean that the one used to sign into iCloud will be required to apply for Apple Card.
– Nimesh NeemaAug 15 at 13:52

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For some of us, we've had an AppleID account longer than iCloud has existed, back since mac.com or me.com, or from early iTunes accounts where there wasn't any apple email address associated - that leaves us with two disparate IDs [or in my case 4, I have mac.com, me.com, iCloud.com & myOwnDomain.com], all of which are theoretically eligible to use. Like the OP, I use me.com for 'sync', but myOwnDomain.com for purchases [the others are largely unused].
– TetsujinAug 15 at 14:17

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@NimeshNeema Ugh — I fear you may be right. I certainly hope that's not true. It would go against the very logic of having separate IDs for iCloud and iTunes/AppStore.
– oromeAug 15 at 22:23